1. Field of Invention
This invention relates to a surface light source device of side light type, and more particularly, to a surface light source device of side light type, which is capable of reducing loss of light caused in relation to fringing. The present invention is applied to a backlighting arrangement in a liquid crystal display, for instance.
2. Related Art
It is a matter of common knowledge that a surface light source device of side light type provides a thin backlighting arrangement to illuminate a liquid crystal display panel. FIG. 5 is an exploded perspective view showing a liquid crystal display employing the surface light source device of side light type in a backlighting arrangement. Referring to FIG. 5, a liquid crystal display 1 has a liquid crystal display panel 2, a surface light source device of side light type 3 arranged for backlighting in the liquid crystal display panel 2, and a substrate (a drive substrate) 4 mounted with a drive circuit or the like in the liquid crystal display panel 2.
In general, the surface light source device of side light type 3 has a light guide plate serving as a light guide member made of a light guiding material, and a primary light source including a long cylindrical light source to supply illumination light to the light guide plate through a lateral side thereof.
Illumination light emitted from the primary light source is introduced into the light guide plate through an end surface of the light guide plate. The introduced illumination light is refracted and then emitted through one of major surfaces of the light guide plate toward the liquid crystal display panel. Since the primary light source is arranged on the lateral side of the light guide plate, it is easy to give a thin overall structure to the device.
Well-known light guide plates employed in the surface light source device of side light type as described above are classified into a light guide plate of a type, which is approximately uniform in thickness, and a light guide plate of a type, which shows a tendency to decrease a thickness according as the light guide plate becomes more distant from the primary light source. In general, the light guide plate of the latter type emits illumination light more efficiently than the light guide plate of the former type.
FIG. 6 is an exploded perspective view showing a surface light source device of side light type employing the light guide plate of the latter type. FIG. 7 is a sectional view taken along a line A--A in FIG. 6.
Referring to FIGS. 6 and 7, the surface light source device of side light type 3 has a light guide plate 6, and a primary light source 7 is arranged on the lateral side of the light guide plate. The surface light source device 3 further has a reflection sheet 8, a diffusible sheet 9, prism sheets 10, 11 functioning as a light control member, and a protection sheet 12 of low diffusibility. A transparent sheet having no diffusibility may be employed as the protection sheet 12. Otherwise, another transparent sheet may be arranged on the further outside of the protection sheet 12. These components are laminatedly arranged.
The primary light source 7 has a cold cathode tube (a fluorescent lamp) 13 and a reflector 14 surrounding the cold cathode tube. The reflector 13 has an aperture, through which illumination light is supplied toward an end surface 6A of the light guide plate 6. The reflector 14 is made of a regular reflective or diffuse reflective sheet material or the like. The light guide plate 6 having a wedge-shaped section is made of an acrylic material (PMMA resin), for instance, by means of injection molding. The light guide plate receives the illumination light from the primary light source 7 through one end surface serving as an incidence surface 6A.
In the light guide plate 6, illumination light makes a propagation while undergoing repetitive reflection between a major surface (which will be hereinafter referred to as "a slope") 6B, along which the reflection sheet 8 is disposed, and another major surface (which will be hereinafter referred to as "an emitting surface") 6C, along which the diffusible sheet 9 is disposed.
Every time reflection occurs, a component of light incident at an angle of not more than a critical angle is emitted through the slope 6B and the emitting surface 6C. As is well known, the illumination light emitted through the emitting surface 6C is inclined to mainly make a propagation toward the wedge end. This phenomenon is called emitting directivity.
Further, a diffusible surface 6D is formed on the slope 6B. The diffusible surface 6D is provided with diffusibility which steps up from the side of the incidence surface 6A toward the wedge end. The diffusibility is provided by coating the slope with diffusible ink containing a pigment consisting of magnesium carbonate, titanium oxide or the like, for instance. The degree of diffusibility may be adjusted according to a quantity of diffusible ink coated on the slope.
The slope 6B may be also provided with the diffusibility by matting (roughening) the slope 6B, instead of coating the slope with the diffusible ink. In this case as well, such diffusibility as steps up from the incidence surface 6A toward the wedge end is provided. Accordingly, matted surface areas of a rectangular shape, for instance, are formed in a distributed state at a certain or random pitch such that the density of areas steps up from the side of the incidence surface 6A toward the wedge end.
The light guide plate 6 described above corrects a quantity of output light decreased in the vicinity of the wedge end, and makes the distribution in quantity of output light uniform. It is to be noted that the diffusibility described above is not so intensive as the light guide plate is allowed to lose the emitting directivity. That is, even if the diffusibility is provided to the slope 6B, the illumination light emitted through the emitting surface 6C is inclined to mainly make a propagation toward the wedge end.
The reflection sheet 8 is made of a sheet-like regular reflective member consisting of metal foil or the like, or a sheet-like diffuse reflective member consisting of a white PET film or the like. Illumination light leaking out of the slope 6B is incident again onto the light guide plate 6 after having been reflected by the reflection sheet 4, resulting in improvement of efficiency of illumination light utilization.
A variety of sheet-like components are arranged parallel to the emitting surface 6C of the light guide plate 6. In this specification, these sheet-like components will be generally referred to as "emission-side additional sheets".
The prism sheets 10, 11 are arranged as the innermost emission-side additional sheets to correct the emitting directivity of the light guide plate 6. The diffusible sheet 9 prevents the diffusible surface 6D on the slope 6B from being visibly observed from above the emitting surface 6C, and also makes highlight and shadow or the like in each part of the light guide plate 6 illuminated with the illumination light less noticeable.
The diffusible sheet 9 diffuses the illumination light emitted through the light guide plate 6. The prism sheets 10, 11 are made of a light-transmitting sheet material such as polycarbonate. In each prism sheet, a surface (an outside surface) with its back to the light guide plate 6 is formed as a prism surface. The prism surface is composed of a large number of projections which are triangular in sectional shape and run substantially parallel in one direction. In this case, the projections on the inside prism sheet 10 are oriented so as to run parallel to the incidence surface 6A, while the projections on the prism sheet 11 are oriented so as to run in a direction orthogonal to the incidence surface 6A.
The prism sheets 10, 11 correct a main emitting direction of output light with a slope of each projection so as to emit output light in a frontal direction of the emitting surface 6C. It may be also possible to use a so-called double faced prism sheet having both surfaces respectively serving as prism surfaces.
In general, the surface light source device of side light type employing the wedge-shaped light guide plate and the prism sheets described above emits output light in the frontal direction more efficiently than a surface light source device of side light type employing a light guide plate which is substantially uniform in thickness.
The protection sheet 12 protects a surface of the prism sheet 11 from damage or the like, and relieves the directivity of output light corrected by the prism sheets 10, 11 to enlarge an angle of visual field. Accordingly, it is possible to emit the illumination light to a desired extent in the frontal direction of the emitting surface. Consequently, the diffusible sheet 9, the prism sheets 10, 11 and the protection sheet 12 respectively function as illumination light correction sheets to correct the characteristics of output light direction. Most of the emission-side additional sheets serve as the illumination light correction sheets as described above.
On the other hand, the liquid crystal display panel 2 has various components such as a polarizer, a glass substrate and a liquid crystal cell, which are laminatedly arranged. The liquid crystal cell is put between a pair of glass substrates respectively provided with transparent electrodes. A peripheral edge of the liquid crystal display panel 2 is generally subjected to fringing 19 at an appropriate width. For instance, the fringing 19 is provided in the form of a light shielding layer formed by coating the peripheral edge with black ink by means of printing or applying a black tape to the peripheral edge and so on. The light shielding layer restrains illumination light from intruding into the liquid crystal display panel through its periphery and accordingly prevents a reduction in sharpness of image display.
It is to be noted that the number of steps of manufacturing the liquid crystal display panel 2 is increased due to a process of subjecting the liquid crystal display panel 2 to the fringing 19. If the fringing 19 is applied to the surface light source device of side light type 3, instead of the liquid crystal display panel 2, it is possible to avoid an increase of the number of steps of manufacturing the liquid crystal display panel 2.
Further, if the fringing 19 is not applied to the liquid crystal display panel 2, it is possible to rationalize the distribution of works in the whole steps of manufacturing the liquid crystal display 1, and to reduce the number of steps as a whole.
However, the process of subjecting the surface light source device of side light type 3 to fringing leads to a problem of a reduction in efficiency of illumination light utilization. The reason is that when the surface light source device of side light type 3 is subjected to fringing such as to trim a luminant portion thereof, illumination light is absorbed in an area subjected to fringing, and this phenomenon affects a luminant portion (which does not require shielding) surrounded with the area subjected to fringing, resulting in a reduction of luminance in an emitting area. On the other hand, when the liquid crystal display panel 2 is subjected to the fringing 19, the above phenomenon does not occur, and there is no possibility that the fringing 19 provides dark display.
The above conditions are similarly observed also in cases other than the display employing the liquid crystal display panel as a substance arranged for backlighting. That is, it is preferable that the luminant portion of the surface light source device itself may be subjected to fringing, instead of the substance arranged for backlighting.